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1 DESIGN MANUAL FOR ROADS AND BRIDGES VOLUME 2 SECTION 2 HIGHWAY STRUCTURE DESIGN: (SUBSTRUCTURES, SPECIAL STRUCTURES AND MATERIALS) SPECIAL STRUCTURES PART 12 BD 31/01 THE DESIGN OF BURIED CONCRETE BOX AND PORTAL FRAME STRUCTURES SUMMARY This document sets the Standard requirements for and gives advice on the design of buried concrete box and portal frame structures of precast and cast in-situ construction up to 15 metres long from abutment to abutment and with up to 11m of fill above the roof slab. In addition the Standard gives requirements for construction, installation and procurement of such structures. INSTRUCTIONS FOR USE This revised Standard is to be incorporated in the Manual. 1. This document supersedes BD 31/87 and SB 3/88, which are now withdrawn. 2. Remove existing contents page for Volume 2 and insert new contents page for Voluem 2 dated. 3. Remove BD 31/87 and SB 3/88, which is superseded by BD31/01, and archive as appropriate. 4. Insert BD 31/01 in Volume 2, Section 2, Part Archive this sheet as appropriate. Note: A quarterly index with a full set of Volume Contents Pages is available separately from The Stationery Office Ltd.

2 DESIGN MANUAL FOR ROADS AND BRIDGES BD 31/01 THE HIGHWAYS AGENCY SCOTTISH EXECUTIVE DEVELOPMENT DEPARTMENT THE NATIONAL ASSEMBLY FOR WALES CYNULLIAD CENEDLAETHOL CYMRU THE DEPARTMENT FOR REGIONAL DEVELOPMENT NORTHERN IRELAND The Design of Buried Concrete Box and Portal Frame Structures Summary: This document sets the Standard requirements for and gives advice on the design of buried concrete box and portal frame structures of precast and cast in-situ construction up to 15 metres long from abutment to abutment and with up to 11m of fill above the roof slab. In addition the Standard gives requirements for construction, installation and procurement of such structures.

3 Registration of Amendments REGISTRATION OF AMENDMENTS Amend Page No Signature & Date of Amend Page No Signature & Date of No incorporation of No incorporation of amendments amendments

4 Registration of Amendments Volume 2 Section 2 REGISTRATION OF AMENDMENTS Amend Page No Signature & Date of Amend Page No Signature & Date of No incorporation of No incorporation of amendments amendments

8 Symbols and Definitions L L Edge L t Longitudinal Joint Ground Level Longitudinal Joint H h h na L j D L j Water Level Z Y h w X clear X h w L j Figure 1 : Symbols for Typical Box Structure DEFINITIONS The following definitions and abbreviations are used in this standard: BS5400 Part 4 means BS5400 Part 4 as implemented by BD 24 (DMRB 1.3.1) and supplemented by BD 57 (DMRB 1.3.7). BD 37 means BS5400 Part 2 as implemented by BD 37 (DMRB 1.3). Longitudinal means perpendicular to the walls. Transverse means parallel to the walls. Ground Level means finished carriageway level, or the temporary ground level on which traffic can run during construction. Hard material means material which requires the use of blasting, breakers or splitters for its removal. Cover means the depth of fill between ground level and the top of the roof. Abutment means an end wall to which horizontal earth pressure loads are applied. Longitudinal Joint means a break in the lateral structural continuity of the structure HAUDL/KEL Combination means the combination of the Uniformly Distributed Load and Knife Edge Load described for HA loading in BD 37. Traction means the longitudinal live load described in BD 37 arising from braking and acceleration of vehicles. Differential Temperature means variations in temperature through a section at a given moment in time. Temperature Range means the difference between the highest and lowest mean temperatures that an element is likely to sustain during its life or the specified return period. Self Equilibrating Stress means the stresses occurring in a simply supported or continuous member as a result of a non-linear strain diagram being imposed on a section in which plane sections remain plane. 3

9 Symbols and Definitions Volume 2 Section 2 SDL is an abbreviation of Superimposed Dead Load. SLS is an abbreviation of Serviceability Limit State. ULS is an abbreviation of Ultimate Limit State. 4

10 Chapter 1 Introduction 1. INTRODUCTION 1.1 SUMMARY This document brings up to date the design requirements for buried concrete box and portal frame structures of precast segmental and in-situ construction. It sets the Standard requirements for these structures and gives advice on their design. The design rules are more comprehensive than those in BD 31/87 and have been written taking into account comments received over a period of years. Structures with depths of cover up to 11m are included in the scope of this Standard which also gives the requirements for construction and installation and the procedures to be followed when procuring these structures which permit the Contractor to choose a proprietary structure that meets the Overseeing Organisation s requirements. 1.2 EQUIVALENCE The construction of buried concrete box and portal frame structures will normally be carried out under contracts incorporating the Specification for Highway Works (MCHW1). In such cases products conforming to equivalent standards or technical specifications of other states of the European Economic Area and tests undertaken in other states of the European Economic Area will be acceptable in accordance with the terms of Clauses 104 and 105 in Series 100 of MCHW1. Any contract not containing these Clauses must contain suitable clauses of mutual recognition having the same effect regarding which advice should be sought. 1.3 SCOPE (a) This document sets the Standard for buried box structures and portal frames for which: (i) (ii) the depth of cover measured from the finished ground level to the roof of the structure is up to 11.0m the length of the structure between the inside faces of the outermost walls (measured perpendicular to the walls) is greater than 0.9m and up to 15.0m. (b) (c) (d) The structures covered by this document are precast or in-situ boxes or portal frames, constructed of reinforced or prestressed concrete. The structures can be single or multi-span with roof slabs that are either integral with, or pinned to, the abutments. Structures not covered by this document include: (i) (ii) (iii) (iv) (v) (vi) (vii) Structures with inclined abutment walls. Structures with abutment walls that are pinned at both top and bottom. Any structure that would behave as a mechanism when not backfilled. Structures with moving bearings at either abutment. Proprietary precast arch structures. Structures with piled foundations. Structures with walls constructed of contiguous or similar piling. (viii) Structures with reinforced earth abutments. This document gives guidance on the installed structure but does not address the loads imposed during construction by thrust boring or jacking structures into place. 1.4 ENVIRONMENTAL CONSIDERATIONS The Overseeing Organisation s requirements for environmental design shall be taken into account in designing buried box and portal structures. Volume 10 of DMRB (Environmental Design) gives advice on the use of underpasses by multiple species of small mammals and fish. It illustrates various forms of culvert design to facilitate free passage of these species. Often these considerations are fundamental to the determination of the span, headroom, cross-section invert and gradient of the structure. 1/1

11 Chapter 1 Introduction Volume 2 Section IMPLEMENTATION This document shall be used forthwith on all schemes for the construction and improvement of trunk roads, including motorways, currently being prepared, provided that, in the opinion of the Overseeing Organisation this would not result in significant additional expense or delay progress. Design Organisations shall confirm its application to particular schemes with the Overseeing Organisation. In Northern Ireland, the use of this Standard will apply on the roads designated by the Overseeing Organisation. 1.6 MANDATORY REQUIREMENTS Sections of this document which form mandatory requirements of the Overseeing Organisation are highlighted by being contained within boxes. The remainder of the document contains advice and enlargement which is commended to designers for their consideration. 1/2

12 Chapter 2 Design Principles 2. DESIGN PRINCIPLES 2.1 LIMIT STATES (a) (b) In this Standard, limit state principles have been adopted for the design of the structural elements and the foundations. Both an Ultimate Limit State and a Serviceability Limit State are considered. The Ultimate Limit State (ULS) is that represented by the collapse of the structural element concerned. 2.2 DESIGN PRINCIPLES OF STRUCTURAL ELEMENTS The Overseeing Organisation s requirements for the design of the concrete structural elements are contained in BS5400: Part 4 as implemented by BD 24 (DMRB 1.3.1) and supplemented by BD 57 (DMRB 1.3.7) except as specified otherwise by this Standard. These documents are hereafter referred to collectively as BS5400 Part 4. (c) (d) (e) The Serviceability Limit State (SLS) is that represented by the condition beyond which a loss of utility or cause for public concern may be expected and remedial action required. In particular, crack width shall be limited as described in Clause and there shall not be excessive movement at the joints capable of seriously damaging the carriageway above. (See Clauses and 4.2.4) Design loads (Q*) are expressed as the product of nominal loads and the partial safety factor. Design load effects (S*) are expressed as the effects of the product of the design loads (Q*) and the partial safety factor γ f DESIGN PRINCIPLES OF FOUNDATIONS Even when the structural elements are designed to their required strength, the structure as a whole can fail due to overloading of the soil-structure interface or excessive soil deformations. In order to prevent such failures occurring, two situations shall be investigated prior to carrying out the final structural design, to confirm whether or not the proposed geometry and structural form are suitable. (a) Sliding The possibility of failure of the structure by sliding on its base shall be investigated at ULS. (f) (g) (h) Design resistance (R*) is expressed as the nominal strength of the component divided by the partial safety factor γ m. The design load effects (S*) at ULS shall not be greater than the design resistance (R*). In addition, for precast segments at SLS, the vertical deflection of the roof slab under live loads (including foundation settlement) shall not be greater than the limiting value given in the Standard. (b) Bearing Failure and Settlement of the Foundations. The maximum net bearing pressure under the base of the structure under nominal loads shall be checked against the safe bearing pressure of the foundations to ensure that there is an adequate factor of safety against bearing failure of the foundation and to prevent excessive settlement and differential settlement. (i) The design life of buried concrete box and portal frame structures shall be 120 years. 2/1

13 Chapter 2 Design Principles Volume 2 Section LOADS (a) (b) The Overseeing Organisation s requirements for Loading are contained in BS5400: Part 2 as implemented by BD 37 (DMRB 1.3), except as specified otherwise by this Standard. These documents are hereafter referred to collectively as BD 37 The following loads (which are described more fully in Chapter 3) shall be used in the design: (i) (ii) Permanent Loads Dead Loads Superimposed Dead Loads Horizontal Earth Pressure Hydrostatic Pressure and Buoyancy Differential Settlement Effects Vertical Live Loads HA or HB loads on the carriageway Footway and Cycle Track Loading Accidental Wheel Loading Construction Traffic (c) Combination 4 Permanent loads and Horizontal live load surcharge plus one of the following: (i) Traction (ii) Accidental load due to skidding (iii) Centrifugal loads (iv) Loads due to collision with parapets and the associated vertical (primary) live loads in accordance with BD 37. Details of the load combinations and the associated partial safety factors to be used in the design of the structural elements are given in Table 3.2 at the end of Clause 3.3 Details of the load combinations and associated partial load factors to be used in the design of the foundations are given in Clause 3.4. (iii) Horizontal Live Loads Live Load Surcharge Traction Temperature Effects Parapet Collision Accidental Skidding Centrifugal Load 2.5 LOAD COMBINATIONS The load combinations to be used in the design shall be as given in BD 37. Only combinations 1, 3 and 4 apply to this standard as follows: (a) Combination 1 Permanent loads, Vertical live loads and Horizontal live load surcharge. (b) Combination 3 Combination 1 plus temperature effects. 2/2

14 Chapter 3 Loading 3. LOADING 3.1 PERMANENT LOADS Dead Load The nominal dead load consists of the weight of the materials and parts of the structure that are structural elements excluding superimposed materials described below Superimposed Dead Load (a) (b) (c) The nominal superimposed dead load consists of the weight of the soil cover and the road construction materials above the structure. It shall be applied to the roof of the structure as a uniformly distributed load. The possible effects of positive arching reducing this load shall be ignored. Where consolidation or settlement of the fill adjacent to a buried structure will cause negative arching of the fill above the roof, increased loading will be generated on the roof slab. These effects can be greater if the foundation is on hard material (see definitions). In the absence of reliable estimates of the effects of differential settlement between the structure and the adjacent ground, the superimposed dead load intensities to be applied to the roof of a structure with cover H shall be as follows: (i) (ii) The minimum superimposed dead load intensity shall be taken as γh. The maximum superimposed dead load intensity shall be taken as βγh where: γ is the average nominal bulk density of the fill and surfacing and β is taken from Figure For foundations on hard material only β= (h-8)/3 Values of β β = β 5 β= (h-8)/ Cover Depth H (m) Figure 3.1 3/1

15 Chapter 3 Loading Volume 2 Section Horizontal Earth Pressure (permanent) (a) (b) The nominal permanent horizontal earth pressures applied to the side walls of the structure at a depth D below ground level shall be taken as follows: (i) (ii) (iii) For Combination 1 and 3 loads: A maximum earth pressure equal to K o γd applied simultaneously on both side walls, or A minimum earth pressure equal to 0.2γD applied simultaneously on both side walls For Combination 4 loads with traction: A disturbing earth pressure equal to K a γd acting in the same direction as the horizontal live load, and A restoring earth pressure equal to 0.6γD acting in the opposite direction to the horizontal live load If under the above loading the structure sways in the opposite direction to the applied horizontal live load, this load case need not be considered. For Combination 4 loads with skidding, centrifugal load or parapet collision, when relevant: As (i) or (ii) above to give the most onerous effect. Values of Earth Pressure Coefficients (i) If the backfill properties are not known but the backfill materials comply with the requirements of Chapter 5 the following nominal default values may be used: K min = 0.2 K a = 0.33 K o = 0.6 K p = 3.0 (c) (ii) (iii) If the backfill properties are known, the nominal values of K a, K o and K p may be calculated from BS8002: 1994, as described in Appendix B. However, as the value of K o determined using BS8002: 1994 does not account directly for effects such as compaction pressure, thermal expansion and cyclical loading (strain ratcheting) which can lead to a significant increase in earth pressure, the default value of 0.6 should be used for K o (with = 1.5) unless such effects are taken into account. A minimum earth pressure coefficient of not more than 0.2 should be used where earth pressures are beneficial. Pressures in excess of K o but not exceeding 0.5K p may be used to resist sliding (see Clause (d) (ii) and Clause (c)). Where the structure is constructed in a steep trench, the critical horizontal earth pressures may be applied by the native ground rather than by the backfill (see Clause 5.1.1(d)). This shall be investigated by considering potential failure planes in the native ground close to the edge of the trench Hydrostatic Pressure When appropriate, the effect of hydrostatic pressure and buoyancy shall be taken into account. The increase in pressure on the back of the walls due to hydrostatic pressure at a depth Z metres below water level shall be taken as 10Z(1-K) kn/m Settlement The settlement and differential settlement of the sub-soil under unfactored nominal permanent loads shall be calculated from BS8004 using the site investigation data. Any differential settlement of the soil that is likely to affect the structure shall be taken into account. 3/2

16 Chapter 3 Loading 3.2 LIVE LOADS Vertical Live Loading HA and HB Carriageway Loading The nominal carriageway loading shall be HA or HB Loading as described in BD 37, whichever is the more onerous. (a) (b) HA Loading (i) Where the depth of cover (H) is 0.6m or less, HA loading shall consist of the HAUDL/KEL combination. No dispersion through the fill of either the HAUDL or the HA knife edge load shall be applied. (ii) (iii) For cover depths exceeding 0.6m, the HAUDL/KEL combination does not adequately model traffic loading. In these circumstances the HAUDL/KEL combination shall be replaced by 30 Units of HB loading, dispersed through the fill as described in paragraph (c) below. Account shall also be taken of the single 100kN HA wheel load, (dispersed through the fill as described in paragraph (c) below), where this has a more severe effect on the member under consideration than the loads described in (i) or (ii) above. HB Loading (i) (ii) 45 Units of HB loading shall be applied on structures on Trunk Roads and Motorways. On structures on other Public Highways, 30 Units shall be applied unless a higher value is specified by the Overseeing Organisation. A minimum of 30 Units of HB loading shall be applied to all structures including those that are designated to carry HA loading only. (c) Dispersal of Wheel and Axle loads through the Fill (i) (ii) (iii) (iv) All wheel loads shall be assumed to be uniformly distributed at ground level over a contact area, circular or square in shape, based on an effective pressure of 1.1N/mm 2. Dispersion of a wheel load through the fill may be assumed to occur both longitudinally and transversely from the limits of the contact area at ground level to the level of the top of the roof at a slope of 2 vertically to 1 horizontally as shown in Figure 3.2a. Where the dispersion zones of the individual wheels overlap, they may be combined and distributed jointly as shown in Figure 3.2a (Zone 2). This applies to adjacent wheels on the same axle and to wheels on succeeding axles. As an alternative to the method described in (ii) the effects of a wheel load on the structure may be derived using Boussinesq s theory of load dispersion as given in standard text books on soil mechanics. The Boussinesq Theory states that for an infinite elastic half space the vertical pressure at a horizontal distance R and a depth z from a vertical point load P applied to the surface is given by: 1 + 3P 2πz 2 R z 2 Where however any individual wheel is located close to the edge of the structure such that its 2:1 dispersal zone is curtailed by a headwall, the increase in pressure near to the headwall shall be taken into account. This may be done by assuming that the load is dispersed transversely over the curtailed width of the 2:1 dispersal zone, as shown in Figure 3.2b /3

17 Chapter 3 Loading Volume 2 Section 2 (v) A wheel load not directly over the part of the structure being considered shall be included if its dispersion zone falls over the part of the structure. (d) Dispersal of the Wheel and Axle Loads through the Roof Slab. Where the dispersed width of the wheel or axle at roof level is less than the spacing between adjacent joints (L j ), a further lateral dispersal of the load may be made at 45 o down to the neutral axis of the roof slab (at depth h na ) so that: where: and A single wheel is dispersed over a total width of C+H+2h na An axle is dispersed over a total width of C+(n-1)S+H+2h na n is the number of wheels on an axle S is the wheel spacing h na is the depth from the top of the roof to the neutral axis which may for convenience be approximated to half the overall roof depth. Dispersion through the slab at 45 o cannot occur through a longitudinal joint. The above approach does not account for the distribution properties of the structure itself (see Clause 4.1.1(e)). 3/4

19 Chapter 3 Loading Volume 2 Section Footway and Cycle Track Loading (a) (b) Footway and cycle track loading shall consist of a load of 5kN/m 2 applied over the total area of the footway or cycle track except that this load may be reduced, by a factor of 0.8, to 4kN/m 2 for elements that carry both footway/cycle track loading and carriageway loading. The loading may be assumed to be dispersed at a slope of two vertically to one horizontally from the edge of the load to a total width not greater than twice the distance from the centre of the footway to the nearer headwall unless a more rigorous dispersion analysis is undertaken Accidental Wheel Loading on Edge Members (a) (b) Where the elements of a structure supporting outer verges, footways or cycle tracks are not protected from vehicular traffic by an effective barrier, they shall be designed to sustain the local effects of the accidental wheel loading described in BD 37. Each of the accidental wheel loads shall be dispersed through the fill using the principles described in Clause (c) and (d) and Figures 3.2a and 3.2b. No other vertical live load nor dispersed load from the adjacent carriageway need be considered in combination with the accidental wheel loading Loading on Central Reserves On dual carriageways the portion of structure supporting the central reservation shall be designed for full HA or HB carriageway loading Construction Traffic Under the low cover conditions which prevail during construction, the structure may be subjected to load conditions that are more severe than those experienced in normal service. During the design stage therefore, consideration should be given to the type of construction traffic likely to be relevant at different stages, and details of the live load capacities of the structure under various depths of cover should be recorded on the drawings to ensure that these are not exceeded during construction. Horizontal Live Loads Live Load Surcharge (a) (b) A horizontal live load surcharge shall be applied in conjunction with all vertical live loads. The nominal uniform horizontal pressure (p sc ) to be applied to the external walls of the structure shall be determined from the equation: p sc = K.v sc where K is the value of the nominal earth pressure coefficient from Clause for the wall under consideration and v sc is the vertical surcharge pressure applied behind the abutments as follows: Vertical LL v sc HA Loading 10 kn/m 2 45 Units of HB 20 kn/m 2 30 Units of HB 12 kn/m 2 Footpath & Cycle Track 5 kn/m 2 Accidental Wheel 10 kn/m 2 Construction 10 kn/m 2 or as otherwise determined For between 30 and 45 units of HB the value of v sc shall be linearly interpolated. The same value of nominal live load surcharge with the same partial safety factors and γ f3 shall be applied simultaneously to both external walls except as follows: In conjunction with Combination 4 horizontal live loading (for traction see Diagrams A/4, A/5 and A/6 in Appendix A). For calculating the maximum bearing pressure (see Diagram A/7 in Appendix A). In these cases the live load surcharge pressure shall be applied on one face only to maximise the effect under consideration. 3/6

20 Chapter 3 Loading (c) It should be noted that when the minimum permanent earth pressure is applied on both sides of the structure (Clause 3.1.3(a)(i)) no live load surcharge shall be applied to either wall (see Diagram A/3a and 3b) Traction (a) The structure shall be designed to resist the traction forces (longitudinal live loads) described in BD 37. (f) The traction force shall be applied directly to the roof of the structure over the following widths measured perpendicular to the direction of the traction force. HA traction - a width equal to the notional carriageway lane width given in BD 37 HB traction - a width equal to 3 + C metres. (b) (c) The traction force shall be applied perpendicular to the walls of the structure for precast construction and parallel to the direction of traffic for in-situ construction. HA Traction (i) For structures with cover not exceeding 0.6m, the HA traction force shall be applied in accordance with BD 37. The loaded length for calculating the traction force shall be overall length of the structure in the direction of the force, except where the most onerous effect on the member under consideration occurs with the structure loaded over only part of its length. (g) In-situ boxes and portal frames are very effective in the lateral distribution of traction because of the in-plane rigidity of their roof slabs. For in-situ structures designed on a metre width basis, the traction force may therefore be considered to be distributed transversely through the structure over a width of 2E T, where E T is the distance of the centre of the traction force from the nearer edge of the structure (or from the nearest longitudinal joint) but not less than half the traction width given in (f) above. Alternatively, if consideration is given to the lateral eccentricity of the traction force and the resistance to plan rotation of the foundations and walls, the traction may be considered to be distributed over the full width L j. (d) (e) (ii) For structures with cover greater than 0.6m, no traction force need be considered in conjunction with the 100kN HA wheel load, but where, as in Clause 3.2.1(a)(ii), HA loading is replaced by 30 units of HB vertical loading, this shall be applied in conjunction with 30 units of HB traction, with at ULS taken as 1.1. HB Traction HB traction shall be applied in accordance with BD 37 when one or more axles of the HB vehicle are on the structure. All traction forces shall be multiplied by K t before they are applied directly to the roof of the structure where: K t = (L L H)/(L L 0.6) but 1 K t Load Effects Due to Temperature (a) (b) Temperature effects may be neglected where: (i) the cover (H) > 2m and X clear < 0.2L t, or, (ii) the overall length of the structure L L 3m. In all other buried structures the variations in mean temperature (Temperature Range) and temperature gradients within a section (Differential Temperature) shall be applied as given below. The coefficient of thermal expansion (α) shall be taken as 12 x 10-6 per C for concrete except for concrete with limestone aggregates where α may be taken as 9 x 10-6 per C. Interaction between the backfill and the structure due to temperature effects may be neglected (but see Clause 3.1.3(b)(ii)) 3/7

21 Chapter 3 Loading Volume 2 Section 2 (i) Temperature Range The temperature range to be used for both the in-service stage and the construction stages shall be in accordance with BD 37, except that the expansion and contraction of the roof relative to the base of the walls due to thermal effects may be based on the temperature range given below: (ii) The effects of temperature range shall be considered at SLS. They need not be considered at ULS for buried structures because expansion or contraction beyond the serviceability limit state up to the ultimate range will not cause collapse of the structure. Differential Temperature Temperature Range For For Expansion Contraction Box structures 10 C to T max 10 C to T min Precast Portal 0 C to T max 20 C to T min frames In-situ Portal 10 C to T max 30 C to T min frames where T max and T min are the maximum and minimum effective temperatures of the roof given in Table 3.1. The datum temperatures for box structures given above are based on the assumption that the temperature of the base will not be greater than 10 C when the roof is at its coldest and not be less than 10 C when the roof is at its hottest. The datum temperatures for precast portal frames assume that when a precast segment is installed its effective temperature will not be less than 0 C nor more than 20 C. For insitu portal frames these figures are both increased by 10 degrees to compensate for effects due to the heat of hydration in the roof and subsequent shrinkage. The effects of temperature gradients within a section (differential temperature) shall be applied to the roof slab only, but the effects of the resulting flexure on other members shall be considered. The differential temperature gradients within the roof section for both the inservice and construction stages shall be taken from Table 3.1. For structures with both X clear < 0.2L t and H > 0.6m, the moments and shears calculated using the temperature differences and dimensions given in BD 37, Figure 9, Group 4 should be multiplied by the reduction factor η given in Table 3.1. Simple methods for calculating the nominal moments and shears around a structure arising from the temperature differences in the roof slab taken from BD 37, Figure 9, Group 4 are given in Appendix C. Self equilibrating stresses due to differential temperature (see Definitions ) need only be considered for prestressed concrete roof slabs at SLS. The datum temperatures for portal frames may be modified if limits on the temperature at the time of construction or installation are specified. 3/8

22 Chapter 3 Loading Span to Cover Minimum and Differential temperature Width maximum Ratio effective temperature Temperature Reduction difference factor X clear /L t H (m) T min T max η 0.2* All depths In accordance In accordance N/A with BD 37 with BD 37** < 0.2 H 0.6 In accordance In accordance N/A with BD 37 with BD 37** 0.6 < H o C 20 o C From BD 37, 0.5 Figure 9, Group < H o C 16 o C From BD 37, 0.33 Figure 9, Group < H o C 13 o C From BD 37, Zero Figure 9, Group 4 H > 2.0m Temperature effects may be neglected Load effects due to Temperature TABLE 3.1 * Structures for which the maximum clear span X clear is more than 20% of the width L t are considered to be open to the atmosphere and the effects of temperature are therefore taken into account in accordance with BD 37 ** For fill depths greater than 0.2m the temperature differences given in Table 24 of BD 37 for 0.2m of surfacing may be used. For roof slabs less than 600mm thick the resulting fixed end moments may be approximated as 0.5 times the values given in Table C1 of Appendix C. 3/9

23 Chapter 3 Loading Volume 2 Section Parapet Collision The effects of parapet collision loading shall be considered in accordance with BD 37 where such loading is transmitted to the buried structure. Particular care should be taken in the design of segmental structures to ensure that such loading does not lead to the opening of joints between segments. In most cases with segmental construction it will be necessary to design headwalls and parapets which carry large transverse loads, such as vehicle impact or earth pressure, as independent structures which do not transmit these loads to precast units Skidding Loads For structures with cover not exceeding 0.6m, loading due to skidding forces shall be considered in accordance with BD 37. Skidding loads do not need to be considered for structures with cover exceeding 0.6m Centrifugal Loads For structures with cover not exceeding 0.6m, loading due to centrifugal forces shall be considered in accordance with BD 37. Centrifugal loads do not need to be considered for structures with cover exceeding 0.6m. 3.3 LOAD COMBINATIONS AND PARTIAL SAFETY FACTORS FOR THE DESIGN OF THE STRUCTURAL ELEMENTS Values of to be used for the Design of Structural Elements To obtain the design loads for a given load combination, the relevant nominal loads described in Clause 3.1 and 3.2 shall be multiplied by the corresponding value of given in Table 3.2, except that, for the applied loads causing a relieving effect on the element under consideration, the value of shall be taken as 1.0. Where the same nominal values of horizontal earth pressure or live load surcharge are applied simultaneously on both sides of the structure, the same values of (from Table 3.2) shall also be applied to the relevant loads on each side of the structure. (See Diagrams A/1 to A/3 in Appendix A) Values of γ f3 to be used in the Design of Structural Elements (a) The value of γ f3 at SLS shall be taken as 1.0 (b) The value to γ f3 at ULS shall be taken as 1.1 except: (i) (ii) for all relieving effects γ f3 shall be taken as 1.0 For disturbing effects at ULS, where plastic methods are used in the analysis, γ f3 shall be taken as 1.15, as in BS5400 Part Load Combinations to be used for the Design of Structural Elements The loads to be applied simultaneously in any load combination for the design of the structural elements are shown in Table /10

25 Chapter 3 Loading Volume 2 Section LOAD COMBINATIONS AND PARTIAL SAFETY FACTORS FOR THE DESIGN OF THE FOUNDATIONS Sliding The loads to be applied simultaneously for checking the foundations against sliding shall be as follows: (ULS) γ f3 Dead Load Minimum Superimposed Dead Load Buoyancy Traction 1.25(HA)/ (HB) Vertical Live Load associated with Traction Disturbing Earth pressure 1.5/1.2* 1.1 (active) Disturbing Live Load 1.5/ 1.1 Surcharge (active) Relieving earth pressure (see Clause (c)) *The value of 1.5 is to be used with the default value of K and 1.2 for the value of K determined in accordance with BS8002. These loads shall be applied at ULS only, using the values of and γ f3 given above, (see also Diagram A/6 in Appendix A). If the net horizontal force is in the opposite direction to the traction force, sliding need not be considered (except for rotational sliding: see Clause (d) and Clause (d)) Bearing Pressure and Settlement The bearing pressures and settlements under the foundations shall be calculated for the following nominal loads as shown on Diagram A/7 in Appendix A: Dead Load Maximum superimposed dead load Maximum horizontal earth pressure on both sides of the box Hydrostatic Pressure and Buoyancy Vertical Live Loading Live Load surcharge on one side of the box only 3/12

26 Chapter 4 Design 4. DESIGN 4.1 DESIGN OF STRUCTURAL ELEMENTS Structural Analysis (a) The structure shall be analysed as a continuous frame, with pin joints where the walls are not continuous or fully integral with the roof slab or base. The stiffness of any corner fillets may be taken into account. Both ULS and SLS shall be considered. (b) For boxes, an elastic compressible support may be assumed below the base slab except for structures founded on hard material (see definitions). In the former case the foundation shall be considered to be flexible and in the latter case the foundation shall be considered to be rigid. (c) For portal structures, where the moments in the frame are sensitive to the rotational stiffness of the foundations, separate analyses shall be carried out for footings where the foundation shall be considered to be (a) rigid and (b) flexible, (see 4.1.1(b)), to ensure that the effects of the full range of possible foundation stiffnesses are considered. (d) Moments and shears shall be obtained from the analysis at critical positions around the structure. The most critical positions for shear will normally be at a distance d from the inside edge of the fillets (or from the internal corners if there are no fillets) and both shear and coexisting moment shall be calculated at these and other critical positions, see also Clause (e) Analysis by the Unit Width Method (i) In most situations it will be adequate to analyse the structure on a metre strip basis using a two-dimensional frame or similar. For structures where the dispersal zones of adjacent wheels overlap as shown in Zone 2 in Figure 3.2a, it will normally be adequate to base the load per metre width due to vertical live load on the dispersion widths determined in accordance with Clauses 3.2.1(c) and (d). (f) (g) (ii) For structures with shallow fill, where the dispersal zones of adjacent wheels do not overlap (and for the single HA wheel load), the above method may lead to unacceptably conservative results. This is because no account is taken of the lateral load-distribution properties of the structure itself. A more realistic distribution width for calculating the live load effect per metre in this situation may be found by using the Pucher Charts, or the method for the Distribution of Concentrated Loads on Slabs given in BS8110, or by other rigorous methods. It should be noted however that in an elastic analysis, an individual HB wheel cannot be distributed over a width significantly greater than the wheel spacing because of the effects of adjacent wheels. Also, a single dispersed wheel load which is narrower than the segment width (L j ) cannot be distributed over a width wider than the segment width. If a three dimensional model is used consideration shall be given to the interaction of live loads in adjacent lanes as described in BD 37. Portal frames shall be designed for the more onerous effects resulting from assuming that: (i) the base of each wall is fully restrained against horizontal movement, (ii) the base of each wall is restrained longitudinally by a horizontal force not exceeding the frictional resistance of the footing under that wall (see Clause 4.4.2). Where the frictional resistance of the footing is adequate to restrain the wall against horizontal movement then only case (i) need be considered. 4/1

27 Chapter 4 Design Volume 2 Section Skew (a) (b) (c) (d) Precast units shall be rectangular in plan and skew effects can therefore be neglected for precast construction except that any special skewed edge units shall be designed separately. In-situ skewed structures which are long (transversely) relative to their span may be designed on the basis of either the square or the skew span. For structures designed on the basis of the square span however, structural elements within a width of Xsinθ from the edge of the structure shall be designed on the basis of the skew span. Alternatively skewed in-situ structures may be analysed by more rigorous methods such as a three dimensional computer analysis. In skewed, in-situ structures the line of thrust of the horizontal earth pressure forces on one abutment is offset laterally from the line of thrust of the earth pressure forces on the opposite abutment. This results in a plan torque which will be resisted by torsional friction on the base, and, if this is insufficient (and the skew is not too great), by a build up of passive pressure towards the obtuse corners of the abutment walls. (i) (ii) If the line of thrust of the earth pressure forces on one wall passes within the middle third of the other wall (that is if L L tanθ < L j /6) this plan twisting effect may be ignored. If L L tanθ L j /6 and the applied torque (T q ) is greater than the frictional resistance torque of the base (T R ), consideration shall be given to resisting the unbalanced torque by increasing the horizontal earth pressure on the walls towards the obtuse corner. In this case, the maximum earth pressure anywhere on the wall at ULS at depth D (excluding live load surcharge) shall not exceed 0.5γDK p and the structural elements must be designed to resist this increased pressure. (iii) It should be noted that on skewed structures where the line of thrust from one abutment passes close to, or outside, the obtuse corner of the opposite abutment, passive pressure will tend to increase, rather than resist the tendency of the structure to rotate in plan and the danger of failure due to rotational sliding needs to be carefully examined Stages to be Analysed Three stages shall be considered: (i) (ii) (iii) The completed structure backfilled up to the top of the roof. The structure backfilled to an intermediate level between roof level and finished surface level, at which it is proposed to use the structure for construction traffic. The structure, fully backfilled, in service Load Cases to be Considered (a) (b) Each element of the structure shall be designed for each of the three stages listed above, using the most onerous of the following Combinations 1 and 3 effects: (i) (ii) Permanent loads with maximum or minimum dead load surcharge (excluding differential settlement in Stages i and ii) Maximum or minimum horizontal earth pressures (iii) The appropriate Combination 1 and 3 live loads positioned to give the most severe effect to the element under consideration. (iv) Temperature effects (Combination 3 only) In addition, for the In Service stage (Stage iii of Clause 4.1.3), the structure shall be designed for the most onerous of the following Combination 4 effects: 4/2

28 Chapter 4 Design (c) (i) (ii) (iii) (iv) Permanent loads with maximum or minimum dead load surcharge. The Combination 4 horizontal earth pressures described in Clause (a) (ii) or (iii). Either traction, skidding, centrifugal or parapet collision loading. The associated Combination 4 vertical live loads positioned to give the most severe effect to the element under consideration. For Combinations 1 and 4 with traction, the load cases to be applied for the design of the structural elements are shown diagrammatically in Appendix A diagrams A/1 A/ SPECIAL REQUIREMENTS AT THE SERVICEABILITY LIMIT STATE Crack Control In Combination 1 at SLS, crack widths shall be limited in accordance with BS5400 Part 4, except that where the cover (H) is greater than 0.6m, the crack width should be checked for 30 Units of HB rather than HA loading Early Thermal Cracking Early thermal cracking need not be considered for precast segmental construction where the segments are monolithic and of 3m length (L j ) or less. For other buried structures the requirements for the control of early thermal cracking are as specified in BS5400 Part 4, BD 28 (DMRB 1.3) and BD 37 (DMRB 1.3), except that the horizontal steel required to resist early thermal cracking need not be placed outside the primary longitudinal reinforcement. Guidance is also given in BA 57 (DMRB 1.3.8) Deflection (a) In precast construction, and in in-situ structures with longitudinal joints that do not comply with (c)(ii), the net vertical deflection at the midspan of the roof under the combined effects of the elastic deflection of the structure and the short-term settlement (b) (c) of the foundations under the application of vertical live loads at SLS shall be less than 0.015H. This limitation is required to prevent the occurrence of excessive movements at longitudinal joints in structures with low covers, which can seriously damage the overlying carriageway. Where an assessment of the live load deflection of the roof is required, it will be sufficiently accurate to estimate the midspan deflection of the roof using the empirical formula: = 20M max X 2 /h 3 metres where X is the effective span in metres, h is the overall depth of the roof in millimetres and M max is the maximum free span moment in knm/m in the roof due to vertical live load only, at SLS. The free span moment is calculated assuming the roof slab to be simply supported over its effective span (X). The foundation settlement at SLS shall be taken to be the nominal live load settlement derived as in Clause Longitudinal Joints (a) (b) (c) The structures shall be designed to accommodate all differential movements or to resist the forces set up by such movements. In precast construction the joints between the segments shall be designed to accommodate the anticipated settlements. The joints need not be designed to transfer load between the segments. In most cases, where cast in-situ construction is used, the structure acting as a deep beam is capable of accommodating curvatures induced by differential settlements and longitudinal joints should be avoided where possible, for reasons of durability. Where, however, the predicted movements are so large that articulation in the structure is required, the longitudinal joints shall be designed either: 4/3

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